Equivalent airspeed

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Equivalent airspeed (EAS) is the airspeed which represents the same dynamic pressure at altitude that would be generated by flying at the corresponding true airspeed (TAS) at sea level. It is useful for predicting aircraft handling, aerodynamic loads, stalling etc.

EAS = TAS \times \sqrt{\frac{actual\ air\ density}{standard\ air\ density}}

Where 
standard air density is 1.225 kg/m³.

EAS can also be obtained from the aircraft mach number and static air pressure.

EAS ={a_{sl}} M_a \sqrt{P\over P_{sl}}

Where 
asl is the standard speed of sound at 15 °C (661.47 knots)
Ma is Mach number,
P is static air pressure,
Psl is standard sea level pressure (1013.25 hPa)


Combining the above with the expression for Mach number as a function of impact and static pressures gives, for subsonic compresible flow:

EAS={a_{sl}}\sqrt{{5P\over P_{sl}}[(\frac{q_c}{P}+1)^\frac{2}{7}-1]}

Where 
qc is impact pressure


At sea level EAS is the same as calibrated airspeed (CAS). At high altitude, EAS may be obtained from CAS by correcting for compressibility error.

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